A series of relatively rigid molecules, from the endoethenoripavine, endoethanooripavine, and benzomorphan families of opioids known to bind to opiate receptors from rat brain cerebrum homogenates and neuroblastoma-glioma hybrid cells, were converted to ligands potentially capable of irreversible interaction with those receptors. At least one of them has been found to act as an irreversible binding ligand for the mu opiate receptor and is being used to differentiate the mu receptors, known to mediate analgesia, from other opiate receptors.